Flexographic printing is a method of direct rotary printing that uses a resilient relief image in a plate of rubber or photopolymer to print articles such as cartons, bags, labels, or books. Flexographic printing has found particular application in packaging, where it has displaced photogravure and offset lithography printing techniques in many cases.
Many methods of forming relief images are known in the graphic arts. Generally, photosensitive elements comprising an ablatable mask layer over a photosensitive polymer layer can be made into articles bearing relief images without the use of a photographic negative (graphic arts film) or other separate masking device. These photosensitive elements are formed into relief images by first imagewise exposing the element with laser radiation (generally infrared radiation directed by a computer) to selectively remove the mask layer in the exposed areas, and then overall exposing the element with actinic radiation (typically UV radiation) to cure the photosensitive layer in the non-masked areas. The remaining areas of the mask layer and the non-hardened portions of the photosensitive layer are then removed by one or more liquid developing processes.
Examples of flexographic printing precursors are described for example in U.S. Pat No. 5,262,275 (Fan), U.S. Pat. No. 5,703,310 (Van Zoeren), U.S. Pat. No. 5,719,009 (Fan), U.S. Pat. No. 6,020,108 (Goffing et al.), U.S. Pat. No. 6,037,102 (Loerzer et al.), U.S. Pat. No. 6,238,837 (Fan), and U.S. Pat. No. 6,759,175 (Daems et al.) and in EP 0295818 (Cusdin).
Commercial flexographic printing precursors can be prepared from Kodak Flexcel® NX Plate that is used with Kodak Flexcel® NX Thermal media and that is from Eastman Kodak Company and Cyrel® Digital Flexo Plate that is available from DuPont.
U.S. Pat. No. 7,279,254 (Zwadlo) describes a method for making an article with a relief image using a removable film.
Other flexographic printing elements are described in U.S. Patent Application Publication 2005/0227182 (Ali et al.) and in copending and commonly assigned U.S. Ser. No. 11/758,042 (filed Jun. 5, 2007 by Zwadlo, Brown, Fohrenkamm, and Stolt) that describes a masking film and method of using it to improve the relief image.
Flexographic printing elements having integral mask layers (that is, ablatable layers) generally require the use of a high-powered laser-equipped imaging that is specifically configured for imaging such elements. In some instances, multiple machines may be needed to vary the thickness of the relief image.
While the quality of articles printed using flexographic elements has improved significantly in recent years, physical limitations related to the process of creating a relief image in the flexographic printing plate remain. For example, it is very difficult to print small graphic elements such as fine dots, lines, and even text using flexographic printing elements. The density of the image is represented by the total area of the various-sized dots in a halftone screen representation of a continuous tone image. In the lightest areas of an image (commonly referred to as highlights), the dots need to become very small. In the traditional flexographic imaging process, the small dots are generally limited to 4%. Due to the nature of the plate making processes, maintaining small dots on a flexographic printing plate is very difficult. In a pre-imaging (or post-imaging) step, the floor of the relief image is set by area exposure to ultraviolet light from the back of the printing element. This exposure hardens the photopolymer to a desired relief depth for optimal printing. Floodwise exposure to image-forming radiation via a mask layer followed by a processing step to remove unhardened (that is, unexposed) photopolymer produces relief dots having generally conical shape.
The smallest of these dots are sometimes removed during processing, which means that no ink is transferred to those areas during printing (the dot is not “held” on the plate or press). Alternatively, even if the smallest dots survive processing, they are susceptible to damage on the rotary printer, as small dots often fold over or partially break off during printing causing either excess ink or no ink to be transferred during printing.
Copending and commonly assigned U.S. Ser. No. 12/183,173 (filed Jul. 31, 2008 by Zwadlo) describes making a relief image on a flexographic printing plate using selective backside exposure to curing radiation. This allows the formation of highlight dots down to 0.8%.